The present invention relates to a screen-change device and, more particularly, to such devices for extruders which continuously treat molten plastics.
Such screen-change devices serve the purpose of removing impurities by replaceable screen elements, from the molten mass prepared in the extruder before it is processed further. The replacement of the screen elements is effected during the continuous preparation process whenever the resistance to flow of the molten mass increases to an impermissible extent as a result of the clogging of the screen elements.
In view of the increasing demands made on the purity of raw plastics, increasingly finer filter meshes are used, which require more frequent replacement of the filters. Accordingly, larger amounts of molten plastic adhering to the change slide and in the region of the screen elements come into contact with the open air and are cracked under contact with oxygen, thereby impairing the quality of the final product. In order to remedy this problem, it has been proposed to provide separate change slides, each equipped with only one screen borehole, which slides are inserted in parallel flow channels. See, for example, West German Unexamined Patent application 28 11 771.
By using a plurality of change slides, the number of leakage points formed by the openings in the housing for each change slide is correspondingly increased.
The openings in the housings are furthermore blocked by sealing rings whose development favors the attachment of melt residues, whereby burned product passes into the filter chambers upon replacement of the filter.
In another known filter device, cooling channels are provided in the changer housing for the sealing off of the change slide (West German Provisional patent 1 554 913).
The cooling action obtained by this measure is, however, slight and furthermore cannot be controlled from the outside since the heat fed to the changer housing does not permit a suitable sealing of the sliding clearance between change slide and changer housing.
Furthermore, in this filter arrangement the screen chambers are utilized by the screen insert only over a part of their length, so that larger amounts of melt residues which adhere to the change slide are also removed.